1. Field of the Invention
The present invention relates to a method and apparatus for producing molded articles of manufacture comprised of thermo-curing materials including thermo-setting and thermo-plastic materials; and more particularly the present invention relates to the production of such articles wherein a meniscus of such material at a boundary to molding surfaces of a mold assembly during the solidification process is controlled and/or adjusted to provide a desired shape to the molded article in a manner which precludes the formation of flash anywhere on the article during the molding process.
2. Description of the Prior Art
Standard designs of conventional reusable molds provide at least one parting line established by the site of separation between top and bottom and/or inner and outer component mold parts. The parting line cannot be sealed against ingress of moldable material particularly liquids from the mold cavity. Therefore, the parting line fills with moldable material during the actual molding operations whereby an unwanted thin web section usually called flash is formed as an integral part of the molded object. For certain applications the parting line flash is of no consequence and can be left in place. For many applications, however, the flash must be removed. This can usually be accomplished by suitable machining operations. Alternative molding techniques and apparatus are needed when satisfactory machining operations cannot be accomplished as when, for example, the molded article comprises elastomeric material.
While not so limited, the present invention is particularly useful for producing molded articles useful in an as-molded-condition without surface modification to the molded article. Certain molded articles must comply with the requirement of a high surface smoothness throughout. A specific class of such molded articles is ophthalmic devices comprising intraocular, intracorneal and contact lenses. Such ophthalmic devices can be made from glass or clear plastic, e.g. polymethyl methacrylate, silicone rubber or hydrogel polymers. In this class of materials, silicone rubber, particularly hydrocarbon substituted polysiloxane offers desirable properties making this material well suited for ophthalmic devices. One recent development in the intraocular lens art provides that a lens made of molded silicone rubber can be folded for insertion through an unusually small incision at the limbal area of the eye.
In the manufacture of a silicone rubber intraocular lens, a two part liquid silicone polymer is thoroughly mixed and a desired quantity is introduced into one part of a mold cavity in a mold block that can be positioned in a cooperative relation with a second part of a mold cavity to form the lens. In the known method, the amount of liquid silicone polymer introduced into the mold cavity was always greater than the amount necessary to form the cured intraocular lens. In this way, it could be assured that the mold cavity is completely filled with silicone material. The excess silicone material extrudes out from the mold cavity when the two mold blocks are forced together under pressure while the curing process is completed. The curing is carried out by heating the molds to a predetermined temperature, about 150 degrees C., for a period of time sufficient to cure the silicone material. Curing is usually completed within about 15 minutes residence time in the mold at the curing temperature. Thereafter, the mold blocks are separated, and the molded lens remaining in one of the mold halves must be separated from a flashing consisting of a thin web of cured silicone rubber protruding from the outer periphery of the intraocular lens. The flashing is removed by tearing the flashing from the lens. The lens is then removed from the mold cavity in the mold block. The edge surface of the intraocular lens from which the flashing has been torn away is rough and irregular because the flashing is separated by applying sufficient force to shear the silicone material at the site where the flashing joins the intraocular lens. The site is a haptic part of the lens which functions to anchor, stabilize and position the lens after placement in the eye of the recipient. The present invention seeks to prevent pathology that may develop due to a rubbing or other trauma from contact over a period of time between the rough and irregular peripheral edge surface of the haptic which is the site where the flashing was torn away from the intraocular lens, and the ciliary sulcus or lens capsule.
In the past, ophthalmic devices were also made from hard, non-elastic materials such as glass or methylmethacrylate by first forming lens blank and then shaping the blank by grinding and other machining operations to impart a desired curvature and finish to the finished lens. Machining a lens blank in this manner was limited to a disc-like hemispherical shape, since the machining operations are performed by rotation of the lens blank. On the other hand, machining of elastomeric materials such as silicone rubber to form an ophthalmic device could not be carried out in this manner because the elastomeric material tears when machining or cutting is attempted. One teaching to overcome this shortcoming can be found in U.S. Pat. No. 3,874,124 wherein a lens blank made from elastomeric material is placed on a support surface and cooled to such an extent that hardness of the elastomeric material is increased so that a machining operation can be carried out in the presence of a liquid interface. However, such a machining of elastomeric material has not met with commercial success and is limited to circular and hemispherical geometrical shapes. After completion of the machining operation on the elastomeric material, it was necessary to eliminate contamination by the liquid interface from the elastomeric material. It was also found that the machined surface of the elastomeric material was hydrophobic and thus resisted wetting by tear fluid whereas the unmachined, molded surface of the lens exhibits good wetting properties.